This invention relates to gear spindles such as may be used to drive roll forming rolls of a steel (or other metal) rolling mill or the like. It will be understood that such gear spindles have many applications and that the following description in the application of a steel rolling mill is merely exemplary.
As shown in FIG. 1, a typical steel mill rolling station includes a pair of rolls R1 and R2 mounted in a roll frame F through which steel forms (e.g., slabs, billets, blooms, bars, structural shapes or the like) are rolled. Rolls R1 and R2 are adjustable toward and away from one another over a limited range so as to vary the spacing between the rolls for carrying out the rolling operation. Typically, the rolls R1 and R2 are driven by a motor M whose output shaft drives a speed reducer SR. The output shaft of the speed reducer drives a coupling which in turn drives a pinion gear having two output shafts S1 and S2 spaced vertically from one another for driving rolls R1 and R2, respectively. Each shaft S1 and S2 has respective centerlines CL1 and CL2 (which are also referred to as pinion centerlines, as will be hereinafter described). Each roll R1 and R2 has a respective roll shaft RS1 or RS2 and each has a respective roll shaft centerline RSC1 or RSC2. As generally indicated at GSC1 and GSC2, gear spindle couplings are provided for transmitting power (torque) from shafts S1 and S2 to roll shafts RS1 and RS2, respectively. In addition, these gear spindle couplings accommodate angular misalignment between the centerlines of the input and output shafts (e.g., misalignment between CL1 and RSC1 and between CL2 and RSC2), and accommodate axial movement between their respective input and output ends (i.e., between shafts S1 and RS1 and between shafts S2 and RS2) upon the spacing of rolls R1 and R2 being adjusted during the roll forming operation. In general, gear spindle couplings can accommodate only small angular misalignment (generally less than 3 degrees and more typically about 11/2 degrees). Further, the gear spindle couplings are subjected to high shock loading as a steel billet or the like encounters the rolls R1 and R2.
As shown in FIG. 2, each gear spindle coupling GSC1 or GSC2 includes a pinion end (driver) coupling PC, a roll end (driven) coupling RC, and a torque transmitting shaft TS spanning between the pinion and roll couplings. As shown in FIG. 2, pinion end coupling PC includes a sleeve 11 adapted to receive shaft S1 and to transmit torque from shaft S1 to torque shaft TS. Likewise, roll end coupling RC has a sleeve 111 adapted to received roll shaft RS1 and to drive roll R1. Torque shaft TS has a spline SP incorporated therein which allows torque to be transmitted, but which also allows for changes in the length of the torque shaft as the spacing between rolls R1 and R2 is varied. The construction and operation of the pinion and roll end couplings and of the spline are well known by those skilled in the art and are not directly part of the present invention. Thus, the pinion and roll end couplings will not be described in detail, except as required to understand the operation and function of the present invention.
Again referring to FIG. 2, pinion end coupling PC has a sleeve gear, as generally indicated at 13, mounted in the outboard end of sleeve 11 for receiving one end of torque shaft TS in a manner as will appear. Sleeve gear 13 has internal gear teeth 15 formed around its inner bore. The sleeve gear 13 is fixedly mounted relative to coupling sleeve 11 and rotates with the coupling sleeve. The end of torque shaft TS coupled to the pinion end coupling PC has a gear 17 on its end having external hub teeth 19 in mesh with internal gear teeth 15 of sleeve pinion 13.
The roll end coupling RC has a similar sleeve gear 113 mounted within sleeve 111 for receiving the other end of torque shaft TS. Sleeve gear 113 has internal gear teeth 115 formed in its inner bore. Sleeve gear 113 is fixedly mounted to sleeve 111 and rotates with the sleeve. The other end of torque shaft TS has a hub gear 117 thereon having external hub teeth 119 in mesh with internal gear teeth 115 of sleeve gear 113. It will be understood that for purposes of this disclosure, sleeve gears 13 and 113 and hub gears 17 and 117 are similar and thus, for purposes of brevity and clarity, only sleeve gear 13 and hub gear 17 will be discussed in detail. However, the disclosure of this invention will apply to both sleeve gears and hub gears.
As shown in FIG. 2, the face width of internal sleeve gear teeth 15 is substantially wider than the face width of external hub gear teeth 19 in mesh therewith for purposes as will appear. Because the sleeve and hub gears operate with a degree of angular misalignment therebetween, the hub gear teeth 19 are typically crowned (as illustrated in FIG. 3). That is, the flanks of the hub gear teeth 19 are crowned so as to accommodate angular misalignment and to minimize backlash (i.e., the difference in thickness between the hub gear and the sleeve gear teeth). Of course, it will be appreciated that it is this backlash and the amount of tooth curvature that allows for the desired angular misalignment between shaft S1 and roll shaft RS1. Further, the tips, roots and chamfers of the gear teeth are crowned to prevent interference at high angles. The amount of curvature used on the hub gear teeth is important to the service life of the teeth. It will be appreciated that too sharp a curvature can cause premature wear, pitting and tooth breakage. A typical crowned hub gear tooth 19 is shown in FIG. 3 and in FIGS. 3A-3C which show various cross sectional profiles of the tooth.
The gear spindle typically has a pressure angle between the teeth of the sleeve gear 13 and the hub gear 17 of either 20.degree. or 25.degree.. These pressure angles allow for a more uniform load distribution over the flanks of the teeth and prevent point contact which undesirably results in high compressive (Hertzian) stresses that leads to premature tooth failures.
In general, there are three types of stresses that the gear teeth of a gear spindle experience, namely,
a. Hertz (compressive) stresses that causes failure from wear.
b. Subsurface shear stress where failure is exhibited from pitting of the gear tooth surfaces and/or from spauling.
c. Root tooth bending stresses which causes the teeth to break at their roots.
The highest stresses are typically located at the contact point between the sleeve and hub gear teeth and at the root of the gear teeth. For gear spindles that operate at light loads and high angles, the tooth design is typically limited by stresses at the tooth contact point (i.e., Hertz stresses) and thus surface hardening of the teeth is employed to improve wear. However, for larger spindles that are highly loaded and that operate at more moderate angles (e.g., 1.degree.-11/2.degree.) of angular misalignment, the gears are typically fabricated of a high strength steel, such as ASTM 4140 or 4340, which is usually sufficient to handle the bending stresses at the root. However, as the angular misalignment range, shown in FIGS. 4-8, of the coupling is increased, (e.g., over 11/20.degree.), the gears are typically fabricated of materials having a higher core hardness and/or the strength of the outer surface must be increased to carry the higher stresses. Such materials may includes Nitralloy 135 & N materials grade carburizing steels such as ASTM 8620, 4320, or 3310. Generally, the depth of penetration of the hardening process is about 1/6 to about 1/5 the tooth thickness. Of course, increasing the hardness of the surface of the gear teeth results in increased wear resistance and improved tooth strength.
In theory, if there were no angular misalignment between the sleeve gear and the hub gear, and if the gear teeth were perfectly machined, there would be 100% contact between the flanks of the sleeve gear teeth and the hub gear teeth. However, due to the nature of gear couplings, they are intended to accommodate certain ranges of angular misalignment. Further, it is not possible to perfectly machine the gear teeth. These factors result in a lower portion of the flanks of the gear teeth in contact with one another and wear increases because of sliding action (instead of rolling or conjugate action) between the "mis-formed" flanks of the gear teeth. As noted above, material selection and surface hardening treatments minimize the wear of the teeth, but distortions due to machining imperfections and due to thermal cycles (which are inherent in surface hardening treatments) contribute to uneven spacing of the teeth on the gears and in tooth profiles.
In the past, it has been known to first form the teeth to a desired nominal size, to then surface harden the teeth, and then to perform a secondary forming operation (e.g., shaving or lapping) so that the tooth spacing and profile will better conform to the desired spacing and profile.
More specifically, it has been heretofore known that carburized external gear teeth for speed reducers, which are ground subsequent to heat treatment, can eliminate tooth forming irregularities and inaccuracies due to subsequent heat distortion from heat treatment processes, thus increasing the strength of the gear teeth with a resultant increase in service life. Gear grinding has long been used as a secondary finishing operation for correcting machining inaccuracies, and for removing heat treatment distortions so as to result in accurately formed external gears that operate quietly and have better strength. However, heretofore, it has not been possible to grind the internal teeth of the sleeve gear of the gear spindle couplings without introducing stress risers at the root fillets of the internal gear teeth. Stress risers significantly weaken the strength of the teeth with a concomitant reduction of the service life of the internal gear teeth.
Heretofore, in forming internal gear teeth in a gear spindle coupling, lapping instead of grinding has been used to correct heat treatment and machining errors. Lapping is usually accomplished by running a set of gears in mesh or by running one gear with a gear-shaped master lapping tool to correct errors in involute profile, tooth spacing, and concentricity. As the gears are run (engaged to operating misalignment) with one another, an abrasive lapping compound is used to remove metal from the profile of the teeth. However, lapping is undesirable not only because it is a time consuming operation, but also because each tooth of the sleeve gear must be individually lapped to properly mesh with its corresponding tooth on the hub gear. Thus, the gears are lapped in pairs and must be maintained in matched sets to obtain the optimal benefits of the process. Thus, such lapped gears are not interchangeable without the loss of their benefits.
As noted, prior to the present invention, it has not been possible to successfully grind internal gear teeth. More specifically, in grinding straight cut gear teeth (i.e., where the gear teeth are parallel to the center axis of the gear), the gear is mounted on a shaft and a pair of twin grinding wheels rotating at high speed and are brought into grinding engagement with the profile of the gear teeth to remove precise amounts of material, thereby to shape the gear profile to a desired shape and to form the teeth on a predetermined spacing. The grinding wheels are mounted on a head, and like a shaper, are reciprocated back and forth across the work. During the reciprocating movement, the gear is rolled past the grinding wheel. Generally, the active surfaces of the grinding wheels are the extreme edges. Thus, it is necessary to provide adequate clearance for the edges of the grinding wheels at the root fillet of the internal teeth of a sleeve gear or the like so as to insure that the active grinding surfaces of the grinding wheels have access to all portions of the tooth profile to be finally formed by the grinding wheels. The necessity of providing sufficient clearance for the grinding wheels has heretofore resulted in introducing unwanted stress risers at the root fillets of the internal gear teeth that significantly reduced the strength of the gear teeth and made the teeth so formed much more susceptible to bending fatigue failures. It is to be understood that grinding wheels could be shaped to allow grinding through the tooth radii. However, grinding into the tooth radii puts tensile stresses into the root from the grinding operation which are also undesirable.
Thus, there has been a long-standing need for a tooth construction (i.e., tooth form) and for a tooth forming methodology for allowing internal gear teeth to be formed, surface hardened (with the resultant thermal distortions), and then to enable the teeth to be ground to a final tooth profile and spacing without forming undesired stress risers in the gear root fillets.